The reforming of hydrocarbons is widely used to upgrade hydrocarbon fractions such as naphthas, gasoline and kerosene, by molecular rearrangement, in the presence of hydrogen and a suitable reforming catalyst, usually promoted with chlorine, to improve the anti-knock characteristics thereof. Hydrocarbon feedstreams upgraded by reforming ordinarily are composed of normal and branched paraffins, naphthenic hydrocarbons and even some aromatic hydrocarbons.
Recently, it has been reported that pollution can be reduced by lowering gasoline endpoint to result in a product endpoint where, in a standard ASTM distillation, 90 volume percent of the gasoline distills below about 270.degree. F. to 350.degree. F. (T.sub.90). Based on this, there have been legislative proposals, particularly in areas of high pollution, to require gasoline to meet a maximum T.sub.90 specification of 300.degree. F. Meeting this T.sub.90 permits only 10% of the hydrocarbons in gasoline to boil above 300.degree. F. A significant boiling range conversion of heavy naphthas will be required to meet this goal.
U.S. Pat. No. 4,812,223 describes hydrocracking a C.sub.5 + naphtha over a noble metal-containing zeolite beta naphtha hydrocracking catalyst. U.S. Pat. No. 3,923,641 discloses the hydrocracking of naphthas using zeolite beta. The reference further discloses that C.sub.5 naphthas, and especially C.sub.7 naphthas, may be selectively hydrocracked to yield a high isobutane-normal butane ratio by contacting the naphtha with zeolite beta within the temperature range of from about 400.degree. to about 550.degree. F. The disclosures are silent on the C.sub.9 + conversion.
U.S. Pat. No. 3,702,818 discloses a hydrocracking process for heavy petroleum feeds utilizing a crystalline aluminosilicate.
U.S. Pat. No. 3,793,192 discloses a process wherein a hydrocarbon feed stream is first catalytically cracked in a cracking zone at high conversion levels and subsequently fractionated to obtain light, intermediate, and heavy fractions. The intermediate fraction is treated by reforming to obtain a high octane product. The reformed product is subsequently blended with the light and heavy fractions to obtain a high-octane gasoline.
U.S. Pat. No. 3,806,443 discloses a process for selective hydrocracking of hydrocarbon feed streams followed by reforming. The disclosure is concerned with contacting a relatively wide boiling range naphtha hydrocarbon (C.sub.5 to 400.degree. F.) under selective hydrocracking conditions suitable particularly for removing the relatively low boiling C.sub.5 and C.sub.6 normal paraffins and no more than a minor amount of C.sub.7 paraffins by selective cracking. The hydrocracking catalyst is characterized as a type B catalyst, that is, a porous solid particulate material having a majority of its pores of a substantially uniform small diameter ranging between 4.5 and about 6.0 Angstrom units.
U.S. Pat. No. 4,647,368 discloses partial hydrocracking over zeolite beta, fractionation of the hydrocracked effluent into a C.sub.4 hydrocarbon stream, a light straight run naphtha and a 200.degree. F.+ stream followed by reforming of the lower octane 200.degree. F.+ stream to achieve a higher quality product.
U.S. Pat. No. 3,847,792 discloses a combination process for making narrow boiling range high octane motor fuel by low severity hydrocracking over mordenite followed by catalytic reforming. The charge stock has an initial boiling range of 100.degree. F. and an end boiling range of less than 450.degree. F.
Although the above prior art proposes various processes for improving the lower octane naphtha fractions, difficulties have been encountered in the implementation.
A conventional reformer usually requires a promoter, usually chlorine, as a catalyst promoter. However, the promoter, which easily finds its way into the hydrogen effluent of the reformer presents a problem when reforming is combined with zeolite catalyzed hydrocracking. The materials used as catalyst promoters, like chlorine (in the form of hydrochloric acid) can be poisonous to the zeolite catalyst of the hydrocracker. Therefore, this hydrogen stream cannot be recycled to the hydrocracker and consequently it would appear to be sufficient to recycle the stream back to the reformer since the hydrogen requirements of the reformer are typically about 7:1 hydrogen to hydrocarbon ratio. However, the hydrogen requirements of the hydrocracker are not insignificant (about 2:1 to 1:1 hydrogen to hydrocarbon ratio) and it is inconvenient and expensive to bring fresh hydrogen (a costly refinery commodity) to this stage of the process.